It was not always so. On the contrary, 11 years ago he was such a good friend that my husband, Costya, and I invited him to our wedding. But for Sergei, who has become an extreme Russian nationalist, the honeymoon with the West is over, and so, therefore, is his friendship with me.
Sergei had unnerved me before with odd, anti-Semitic things he said. But it only became clear how far apart we had grown when he dropped in for a drink a few weeks ago and we ended up having an argument. The conversation began harmlessly enough, with a few jokes, but before I knew it we had plunged into politics.
"It's all the fault of the foreigners," said Sergei.
"The West is to blame."
"Well, yes," I said, "the West has made some mistakes, raised expectations that life after Communism would be easy. Unfortunately Russia has not always seen the best side of the West. You've seen our unscrupulous businessmen, our cheap products in the kiosks: but surely Russians themselves must bear some responsibility for their problems. After all, Yeltsin, the members of the government, are Russian."
"They're traitors," said Sergei. "They have got us into debt with the IMF. The only mistake we Russians have made is not resisting the aggression of the West."
I was stunned. If I had heard that argument from some shaven-headed blackshirt in Alexander Barkashov's Russian National Unity Party, I would not have beensurprised. But Sergei is a highly educated man whose father was a famous writer who advised Mikhail Gorbachev on literary freedom.
In the heady days of perestroika and glasnost, when Russians and Westerners met with excitement, we used to sit in Sergei's flat, speaking French because my Russian was still rudimentary, and Sergei would talk of dreams of knowing the wider world.
"Yes, then I only wanted to listen to Western rock music," he said, when I reminded him. "But now, I get tears in my eyes when I hear the balalaika on the radio."
There is nothing wrong with Russian folk music, but something disturbing about Sergei's new-found appreciation for Stalin. And his hatred-filled form of Russian Orthodoxy seems very far from true Christianity.
Most of all, I was shocked by Sergei's definition of the West. It was, he said, not a place, but a world-view based on respect for democracy (to him, a pejorative word), women's liberation (also negative) and a positive attitude to Jews (very bad indeed).
I said I would like to think of the West as a community of countries where everyone counted, man or woman, black or white, Christian or Jew. He called me a "rootless cosmopolitan".
He said that once England, France and Germany were countries with a strong national identity but "the West" had "colonised" them. Russia was falling, too. The Great Satan was America.
"It is an incredibly corrupt and decadent place. It is not only the enemy of the Arabs but of the whole world. But you mark my words, soon it will be destroyed. The Statue of Liberty will be shattered in a thousand pieces."
"I think you really want to see that, don't you?" I said incredulously, remembering how a few years ago Sergei had gone in great excitement on a business trip to Florida. (He also spent two weeks in my family home in Yorkshire, and ought to know that we in the West are human too.)
"Yes, I do," he said, "I dream of seeing America on its knees."
At that, I stopped debating.I felt stone cold, despite the summer heat.
"Well, I'm sorry," I said, "I came to Russia to learn, to help if I could, I did not mean any harm."
"We do not need your help. It's nothing personal, Helen, but you represent the enemy. You are on a Masonic mission."
He was my friend. Those were his parting words.